This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Systems that optically track the position of a target have many uses, including inventory control, security, movement analysis and virtual reality. Some of these systems, such as the Valve/HTC Lighthouse [Deyle 2015] and the Instant Replay system of Raskar et al [Raskar 2006] use small photosensors on the object to be tracked. One or more fixed location base stations emit time-varying patterns of angularly structured light into the scene. The resulting time-varying light intensity measured at each photosensor is used to calculate the angle of that photosensor with respect to the origin point of that transmitted pattern.
Two orthogonally oriented patterns emitted sequentially from the same base station can then be used to compute the solid angle of the photosensor from that base station. Triangulation to compute the three dimensional position of the photosensor in the scene can then be effected through the use of two or more base stations in different locations, or by placing multiple photo-sensors in different known locations on a shared rigid body to be tracked.
These two systems have the benefit that the target photosensors are small, and therefore can be placed unobtrusively on objects to be tracked, or placed in multiple locations on non-rigid objects, such as the bodies or clothing of people to be tracked. For example, either system can be used to track the position of a wand that is drawing in the air in a virtual reality simulation. In this application a photosensor can be placed at the tip of the wand to be tracked.
In another example, either system can be used to track the position and orientation of a virtual reality head mounted display (HMD). In this application a number of photosensors can be placed on different known locations of the HMD. Once the location of each photosensor has been determined, then a “best fit” rigid body can be readily computed from the measured locations of these individual photosensors.
Each of these two systems suffers from practical limitations in measurable angular resolution, due to limitations on practical optical resolution in different parts of the system. The Lighthouse system requires a linear sweep of the scene by a scanning laser line for each angular dimension to be measured. The specific moment in time during this sweep when the scanning line impinges on a photosensor target is used to compute the angular position of that photosensor in the dimension of the sweep, with respect to the emitting base station.
This approach has a resolution limitation due to the fact that the sweep needs to be fast enough for real-time tracking. The Lighthouse system does a complete measurement 60 times per second. This requires four sweeps (one horizontal followed by one vertical for each of two base stations).
This constraint puts a large burden on the timing circuitry on the receiving end that converts detection time to position. Practically this limitation results in a final positional accuracy within the scene that cannot be smaller than about 10 millimeters, for targets that are on the order of two meters away from the base stations.
The Instant Replay system projects a discrete Gray code pattern out to the scene in ten sequential steps (one per binary bit), to determine angular resolution in each dimension. This system has the advantage over the Lighthouse that angular positional resolution is exponential in time: Only n discrete time steps are required to measure 2{circumflex over ( )}n discrete angular positions.
In practice, Instant Replay uses 10 sequentially projected bit patterns to encode 10{circumflex over ( )}2, or 1024, angular positions. Yet this nominal highest angular resolution is achieved only if the receiving aperture of the photosensor is smaller than the pattern detail size at the finest resolution (lowest order bit) of the projected Gray code pattern.
The measurable angular resolution is therefore limited by the size of the optical aperture formed by the photosensor, which in a practical implementation needs to be large enough to gather sufficient light to guarantee an acceptably high signal to noise ratio. Pattern detail that is smaller than the size of the receiving aperture cannot be accurately detected.